The maintenance of steel bridges centers in preventing or inhibiting corrosion, which becomes a particularly severe problem in areas where salt is used to stop the formation of ice on the roadway. If left unchecked, the corrosion can eventually compromise the structure to the point that it can no longer carry its designed load. Corrosion is dealt with by the use of special paints, some of these (particularly the older ones) having a lead base. The effective life of a paint job depends on the nature of the paint, and the manner of its application, including the preliminary surface preparation. Some presently available paints, properly applied, can be expected to last for 20 years or more. Lower quality of materials and work will shorten the life expectancy considerably.
Maintenance procedures normally involve particle blasting the surfaces of the structure to remove the old paint, and condition the surfaces to receive the new coatings. This is followed by spraying on several coats of various materials in sequence. Both steps produce serious pollution problems. The work is commonly done from scaffolding under the bridge, requiring the blocking off of one or more lanes of roadway. Very little serious attention had been given to the confinement of the blasted materials and the sprayed paints to the immediate work area. This has resulted in the release of vast quantities of dust containing particles of lead and other toxic materials in the surrounding atmosphere. The particles are fine enough to become airborne, and may travel surprising distances before settling out eventually and contaminating the ground. Some areas around the sites of major projects have become practically inhabitable, resulting in extensive liability to the contractor and/or to the states where the work was authorized and done.
In the painting phase of the project, the difficulty has been the release of airborne paint particles that migrate from the work area out to where automobiles passing in adjacent lanes are exposed to an unwanted application of spotty color. Two or three hundred cars requiring repainting at over two hundred dollars each raises the cost of the project considerably. In the usual bridge project, there may well be five successive painting passes over the structure in sequence, with labor representing about half of the cost. Because of the difficulty in preventing these conditions, insurance protecting the contractor and the State is either unattainable, or available only at prohibitive costs.
Removal of the material cleaned from the bridge presents still another problem. The quantity of it can run to hundreds of tons, and collection of it and delivery to a disposal site is costly. Above a certain point in toxic content, the disposal becomes especially costly. At further increase in the toxic concentration, state law may require special disposal techniques at state expense. Oddly enough, a lead content of sixty-five (65) percent or more will eventually provide a sizeable market value to manufacturers equipped to recycle the material to salvage the lead for reuse. The usual work procedures, however, require additional processing to increase the toxic concentration to the point that the recycle value becomes available.
These problems are present in the maintenance of other structures as well. The surfaces of buildings, ships, and many others requiring similar treatment, will produce similar conditions. The present invention is directed at the confinement of all airborne particles to the immediate work area, and to the complete recovery of all blasted materials so that blasting particles can be recycled and concentrated for disposal or reuse.